Embodiments of the present invention relate to radio frequency identification (RFID) technology, and more particularly to techniques for tracking the motion of an RFID tag using signal strength information.
RFID technology has seen widespread use in recent times. RFID tags attached to objects are increasingly being used to identify and track the locations of the associated objects. An RFID tag can be read by a reader without requiring a physical contact or direct line of sight to the tag. With advances in RFID technology, the distance over which an RFID tag can be read has also significantly increased over time. While in the past a reader had to be within a couple of inches from an RFID tag to be able to read the tag, tags can now be read over distances of ten meters or more. This has in turn increased the uses or applications of RFID tags.
An RFID tag generally comprises a memory that may be used to stored identification information and/or information related to the object with which the tag is associated. The information stored by an RFID tag may then be read by an RFID reader. There are various types of RFID tags including active tags, passive tags, semi-active tags, and the like. An active tag comprises a power source on the tag (e.g., a battery) and can transmit radio signals autonomously. A passive RFID tag, on the other hand, has no power source on the tag and requires an external source to provoke signal transmission. A passive RFID tag is generally activated upon receiving radio signals from an RFID reader and transmits signals in response to the activation. The signals transmitted by an RFID tag, either active or passive, are read by the RFID reader and may comprise information stored in the memory of the RFID tag.
More recently, in addition to reading information stored by an RFID tag, readers are also capable of determining the received signal strength indicator (RSSI) for signals read from an RFID tag. The RSSI metric measures the signal strength of the radio signal received from an RFID tag (either active or passive). Theoretically, RSSI is directly proportional to the distance of the tag from the reader generating the RSSI metric. However, in real systems, RSSI does not always decrease linearly with increasing distance to the reader. Further, RSSI is also affected by environmental factors such as the presence of objects (e.g., metal objects) that can interfere with signal reception. The RSSI of a single RFID tag may even change every time a reader reads it. As a result, the location of an RFID tag cannot be accurately determined based just upon RSSIs.